Single Radio Voice Call Continuation (SRVCC) is a Packet Switched (PS) to Circuit Switched (CS) HO mechanism, which is specified by 3GPP e.g. in TS 23.216.
An SRVCC is a service that allows an ongoing Voice over Internet Protocol (VoIP) call in LTE to be transferred to the CS domain. This service makes it possible for a VoIP call over LTE to be continued even if LTE coverage is lost.
An SRVCC can be seen as a PS-to-CS handover of a VoIP call. For performing SRVCC of e.g. a voice bearer to the CS domain, the VoIP telephony session needs to be anchored in the IP Multimedia Subsystem (IMS).
An SRVCC for a UE is typically triggered by an LTE eNB serving the UE, when the signal strength, received by the UE, from the eNB and other potential target LTE nodes drops below a certain threshold, i.e. when the UE experiences unfavorable LTE signal conditions, or ‘is having bad LTE coverage’.
The SRVCC mechanisms specified by 3GPP include two variants of handover from LTE to UTRAN, namely ‘CS only’, where only the voice bearer is handed over in a PS-to-CS HO, and other PS bearers are dropped; and ‘CS and PS’, where the voice bearer is handed over in a PS-to-CS HO and other PS bearers are handed over in a PSHO. The LTE eNB selects what type of SRVCC to request.
There is further a version of the SRVCC, called Single Radio Video Call Continuation (vSRVCC), which is also defined by 3GPP. As the name implies, vSRVCC relates to continuation of multimedia calls, or so-called “video calls”, anchored in IMS. Such video calls may also be referred to as VoIP+video calls, IMS video calls or similar. When using the term VoIP herein, as in VoIP bearer, etc. this also embraces VoIP bearers/conversational bearers of multimedia calls.
According to the current implementation of SRVCC in 3GPP, the process is started by the eNB when the eNB requests a HO to e.g. WCDMA or GSM and indicates in the HO required message that an SRVCC is needed.
One problem with the current implementation is that in order to request an SRVCC for a UE, the eNB needs to be aware of that the UE has an ongoing VoIP session and that this VoIP session is anchored in the IMS. However, a basic design principle for Radio Access Network (RAN) nodes, such as eNB, is that these nodes should be unaware of what different kind of services that the UE is using.
Thus, the current approach has led to some very strange attempts to make it possible for the eNB to request an SRVCC for a UE. For example, it has been decided that the radio bearer that is used for a VoIP call (or video call in case of vSRVCC), i.e. the bearer that can be subject to SRVCC, is the only bearer that can be assigned the Quality Class Indicator QCI=1. No other services are allowed to use this QCI value. According to the 3GPP specifications, the operator is free to use different QCI settings for different services, but in order for SRVCC to work, QCI=1 can only be used for VoIP (including VoIP+video) over IMS services. That is, the SRVCC design is not consistent with the common design principles.